Our broad goals in this research are to study the biological constraints and the role of experience in setting up functionally specialized neural systems in normal development and to study the nature and extent of changes in this process in cases of abnormal development. An important approach in this endeavor is to study changes in brain organization that occur as a function of chronological age and to contrast these with changes linked to specific abilities when age is held constant. The variability occurring in normal development, as in the case of early and late talkers, provides one opportunity to address this issue. The study of abnormal development, as in the case of language impaired (LI) children, children with focal brain lesions (FL children) and children with Williams Syndrome, provides another opportunity to link specific changes in neural development with alterations in specific sensory and cognitive abilities. To this end we will record event-related potentials (ERPs) from over several brain regions making comparisons within and between the cerebral hemispheres in a series of studies designed to assess different aspects of sensory, language and cognitive processing. The specific aims of the proposed series of experiments include: (1) To determine (a) the time course of the normally occurring pattern of cerebral specializations for language processing and its relation to the attainment of specific language abilities, and (b) whether the behavioral alterations in the normal time course of language acquisition that occur in FL children, LI children and children with Williams and Down Syndrome, are associated with specific modifications in the organization of language- relevant brain systems. (2) To assess the hypotheses that (a) different neural systems mediate semantic and syntactic aspects of language processing from an early age, (b) neural systems important in grammatical processing are more vulnerable to early experience than are the systems that mediate semantic processing and these may be abnormally organized in language impaired children and FL children and Williams Syndrome, (c) different neural systems mediate language and non-language cognitive processing at all ages and levels of development, (d) abnormal organization of neural systems associated with processing rapidly presented auditory stimuli is linked with abnormal language acquisition in a subset of LI children, and may also be linked to the sparing of language in Williams Ss.